Understanding Online Partial Discharge Testing
Transcript of Understanding Online Partial Discharge Testing
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Understanding Online Partial Discharge Testing
Sanket BolarMegger USA
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Need for condition based maintenance
• Time based maintenance – Tests done at prescheduled times to assess the condition of the asset
• Example, discharge test on a battery
• Condition based maintenance – Certain parameters associated with the condition of the asset monitored continuously
• Example, online DGA on a transformer
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What is partial discharge?
• Localized electrical discharge that only partially bridges the insulation
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Conventional method of PD measurement
• Mainly employed in factory testing• Measured quantity is called apparent charge • Apparent charge (pC) is the pulse charge
measured at the test terminal because of the charge transfer inside the test object due to partial discharge
• Measuring impedance calibrated by injecting a controlled pulse train
• Asset energized by a test voltage source and subjected to overvoltage conditions for a specified duration
• The apparent charge measured during the course of the test needs to be less than the limit specified in the standards
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Unconventional methods of PD measurement
• Using conventional method not practical onsite because of the presence of noise
• Unconventional methods used for detecting partial discharge onsite
• PD detection based on physical properties • No correlation with apparent charge• The following technologies used –
• RF detection• Acoustic methods• Optical methods• Chemical methods
PD
RFI
Sound
Gases
Light
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Sensor technologies
UHF measurement• PD pulse emits electromagnetic waves• UHF range: 300MHz to 3GHz• Most of the background noise is present in
lower frequencies• Some of the sensors used: Field grading
sensors, window sensors, disc sensors, etc.
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Sensor technologies
Acoustic methods• PD emits sound waves in the ultrasonic range• Measurement range: 10kHz to 300kHz• Sensors: Piezo-electric transducers,
accelerometers, condenser microphones, etc.• PD localization using multiple sensors possible• Difficulties in PD localization: Velocity of
propagation differs with media, reflections and refractions can occur
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Sensor technologies
HF measurements• HFCT uses inductive coupling• PD results in short duration current pulses
which are measured by HFCT connected on the ground of the equipment
• Difficulty: HFCT can also detect current pulses from other sources on the grounding grid
• Transient Earth Voltage (TEV) sensors use capacitive coupling
• Transient voltages develop on the metal enclosure because of internal PD
• TEV sensor can be placed on enclosure to detect PD
• Used mostly on switchgear• Difficulty: External sources can also induce TEV
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PD data for corona discharge
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PD data for corona discharge
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PD data for void discharge
At 5 kV At 9 kV
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PD data for void discharge
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PD data for surface discharge
At 3.5 kV At 6 kV
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PD data for surface discharge
At 160 MHz
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PD data for surface discharge
At 1.5 kV
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PD data for surface discharge
At 9 kV
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Case study I - Bushing on an oil circuit breaker
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Case study II – Surface discharge on cables
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Questions?
• Hope you slept comfortably